North Korean leader Kim Jong Un has said his frontline troops are in a "quasi-state of war" and ordered them to prepare for battle a day after the most serious confrontation with South Korea in years.

South Korea's military on Thursday fired dozens of artillery rounds across the border in response to what Seoul said were North Korean artillery strikes meant to back up a threat to attack loudspeakers broadcasting anti-Pyongyang propaganda.

The North's declaration is similar to other warlike rhetoric in recent years, including repeated threats to reduce Seoul to a "sea of fire", and the huge numbers of soldiers and military equipment already stationed along the border mean the area is always essentially in a "quasi-state of war".

Pyongyang's apparent willingness to test Seoul with military strikes and its recent warning of further action raises worries because South Korea has vowed to hit back with overwhelming strength should North Korea attack again.

Pyongyang says it did not fire anything at the South, a claim Seoul dismissed as nonsense.

Kim ordered his troops to "enter a wartime state" and be fully ready for any military operations starting on Friday evening, according to a report in Pyongyang's official Korean Central News Agency.

The North has also given Seoul a deadline of Saturday evening to remove border loudspeakers that, after a lull of 11 years, have started broadcasting anti-Pyongyang propaganda. Pyongyang says failure to do so will result in further military action. Seoul has vowed to continue the broadcasts.

The North's media report said: "Military commanders were urgently dispatched for operations to attack South Korean psychological warfare facilities if the South doesn't stop operating them."

South Korea's Yonhap news agency, citing an unidentified government source, reported that South Korean and US surveillance assets detected the movement of vehicles carrying short-range Scud and medium-range Rodong missiles in possible preparation for launches.

North Korea said the South Korean shells landed near four military posts but caused no injuries. No one was reported injured in the South either, although hundreds were evacuated from frontline towns.

The loudspeaker broadcasts began after South Korea accused the North of planting land mines that maimed two South Korean soldiers earlier this month. North Korea denies this.

Authoritarian North Korea, which has also restarted its own propaganda broadcasts, is sensitive to any criticism of the government run by Kim, whose family has ruled since the North was founded in 1948. The loudspeaker broadcasts are taken seriously in Pyongyang because the government does not want its soldiers and residents to hear outsiders criticise what they call world-leading human rights abuse and economic mismanagement that condemns many to abject poverty, South Korean analysts say.

North Korea on Thursday afternoon first fired a single round believed to be from an anti-aircraft gun, which landed near a South Korean border town, Seoul said. About 20 minutes later, three North Korean artillery shells fell on the southern side of the demilitarised zone dividing the two Koreas. South Korea responded with dozens of 155mm artillery rounds, according to South Korean defence officials.

South Korea's military warned that North Korea must refrain from engaging in "rash acts" or face strong punishment, according to the Defence Ministry.

South Korea raised its military readiness to its highest level. Joint chiefs of staff spokesman Jeon Ha-kyu told a televised news conference that South Korea is ready to repel any additional provocation.

Escalation is a risk in any military exchange between the Koreas because after two attacks blamed on Pyongyang killed 50 South Koreans in 2010, South Korea's military warned that any future North Korean attack could trigger strikes by South Korea that are three times as large.